In order regulate the isochronism of a timepiece regulating assembly, in particular of a sprung balance, it is known to act on the active portion of the balance spring, which is attached at the inner end thereof to a collet secured to the balance, and at the external end thereof to a balance spring stud, secured to a balance cock which is secured to the plate of the movement. In order to alter this active portion, and in particular to obtain a shorter length than the total length defined by the place where the balance spring is set in the stud, an index is used, which may be an index provided with two pins, or an index comprising a balance spring boot carrying the two pins. These two pins define two parallel, polished shoulders on which the balance spring can rest and slide with minimum friction. At a given time, if the balance spring is resting on one of the pins, the active portion thereof is the portion comprised between the place where it is pinned up to the collet, on the one hand, and where it rests on the pin, on the other hand. If the balance spring is not resting on either of the pins, its active portion is equal to its total length. If the pins are very far apart, this creates a losing rate, and the variation amplitude is large, which is not conducive to a regular rate. If they are too close to each other, this creates a gain, and there is a risk of the balance spring being deformed, which is worse.
Since the operator cannot see the relative positions of the balance springs and the pins, he has to proceed by trial and error, or use empirical adjustments, for example adjusting the pins when the balance is at rest, so that the balance spring is centred relative to the pins, allowing play of a half-thickness of the balance spring on both sides. However, like any empirical adjustment, this does not optimise operation, but simply constitutes an adjustment which conventionally considered acceptable. Moreover, this positioning is theoretical, and difficult to carry out because of the impossibility of introducing gauges or suchlike to regulate the spacing between the pins.
The situation is complicated by the combination of possible adjustments to the balance spring stud and the pivoting of the index, in particular, and it is difficult for the operator to control what is actually happening to the balance spring. It is thus more advantageous for the operator to be able to use means informing him of the exact position of the balance spring, in order to perform the operations of centring and adjusting the beat of said balance spring.
CH Patent No 408787 in the name of Vuachet discloses a balance spring boot, which is formed by a plate folded into a V-shape, resting via edges on top of a radial hole in an index. This plate includes a slot for the passage of the spring, the edges of the slot acting as pins for limiting the beat of the balance spring. The operator can see the balance spring through the radial hole in the index, but cannot tell its position relative to the edges of the slot, which are concealed by the structure of the plate, and which are not visible through the hole in the index.
CH Patent No. 40792 in the name of Beyner discloses an index that includes two moving fingers which hold the balance spring. The latter is visible from above between the two fingers. However this is not a moving balance spring boot, but an index body whose position is fixed, apart from the pivoting thereof relative to the axis of the balance spring, and it is not possible to act on the relative angular position between the pins and the spring.